The invention relates to a magnetic bearing for magnetically centering a moving body relative to a fixed body with respect to three axes and preferably for controlling tilting of the moving body.
Magnetic centering of one body relative to another body with respect to a given axis can be passive or active, depending on whether the centering magnetic fluxes are generated passively by permanently magnetized means or at least in part actively by an appropriate choice of the amplitude of an excitation current applied to windings. This is known in the art.
For physical reasons, a body cannot be passively centered relative to another body with respect to three non-coplanar axes.
For a given level of centering performance, the use of permanent magnets has the advantage of minimizing the electrical energy required for centering, compared to a configuration with no magnets.
In practice, the body which is to be centered relative to a fixed body can have various degrees of freedom relative to the fixed body. For example, the moving body may be a rotor which rotates continuously or non-continuously about an axis of rotation which often coincides with one of the three centering axes. A configuration like this is of great practical importance, especially in the space field, in momentum wheels or in reaction wheels.
However, to complement such rotation, it may be necessary to control tilting about one or more axes transverse to the rotation axis. Thus in the field of satellites it is beneficial to be able to incline the rotation axis of a momentum or reaction wheel, for example to contribute to satellite attitude control.
Magnetic bearings with the facility for tilting have already been proposed. The document W089/12178 may be cited in particular.
However, as a general rule, magnetic bearings in connection with which the facility for tilting has been mentioned rapidly lose their centering capability when the tilt exceeds angles of the order of one degree.
The object of the invention is to provide a magnetic bearing (sometimes referred to as a magnetic suspension) for centering a first body, which is mobile in tilting about a tilting center, relative to a second body, which system allows relative tilting through at least 5xc2x0, which tilting is significantly greater than that allowed by prior art magnetic bearings and can in particular reach or even exceed tilting angles of plus or minus 15xc2x0, without compromising centering performance with respect to three non-coplanar axes and using dedicated components of compact geometry and low power consumption implying only a moderate increase in the weight of the moving body. A subsidiary object of the invention is to provide a magnetic bearing which can also control tilting in the range of movement concerned, which is through at least 5xc2x0 or even 15xc2x0 or more.
To this end the invention proposes a magnetic bearing for centering a first body, which is mobile in tilting within an angular range of movement of at least 5xc2x0 about a center of tilting, relative to a second body having a reference axis passing through the center of tilting, the magnetic bearing including:
a hollow outer part at least part of which is made from a ferromagnetic material which is attached to the first body and which has an inside surface whose shape is a portion of a sphere whose center is substantially coincident with the center of tilting and which extends around a mobile reference axis having an inclination which can be zero to the reference axis on either side of a transverse plane which is perpendicular to the reference axis and passes through the center of tilting,
an inner part which is attached to the second body and which includes two separate members which are disposed on respective opposite sides of the transverse plane, which are separated in the direction parallel to the reference axis by a space having a reluctance adapted to prevent the flux lines generated by the group of windings of one of the members crossing this space, which each have a group of windings including at least the specific windings of the ferromagnetic areas and which each include a plurality of (at least three) ferromagnetic areas which are offset angularly about the reference axis, which each define in conjunction with the inside surface of the hollow outer part two air-gaps offset relative to the reference axis and which are each provided with a specific winding adapted to generate magnetic flux lines closing across the two air-gaps, and
an excitation circuit for selectively applying excitation currents to the windings of the group of windings of each member to generate magnetic fields in the air-gaps adapted to center the hollow outer part relative to the inner part in directions transverse to and parallel to the reference axis.
Thus there is a very compact inner part on which a few windings are mounted, possibly with a permanent magnet inside the space between the two separate members. The overall size and weight are therefore low. Around this inner part is a hollow outer part whose inside surface is the shape of a hollow sphere (which is why the magnetic bearing of the invention can usefully be referred as a ball joint bearing), so that the air-gaps defined therewith, at a distance from the aforementioned transverse plane, are inclined relative to the reference axis and can contribute to the generation of centering forces parallel to the reference axis.
As just indicated, the space between the two separate members of the inner part can be occupied by a magnet which is permanently magnetized in a direction parallel to the reference axis. The magnet therefore generates magnetic flux lines continuously without consuming electrical energy. On the other hand, no flux lines generated by an electrical current in any of the windings pass through the magnet.
However, another situation of practical importance is that in which this space is a free space forming a large fixed air-gap, i.e. one which does not contain any solid material, with the possible exception of a non-ferromagnetic connecting member for fastening the two members together. This space is filled with vacuum or with air, depending on the environment in which the magnetic bearing is located.
In conjunction with the air-gaps and a ferromagnetic portion of the hollow outer part and the associated specific winding, each ferromagnetic area defines a magnetic actuator. The various magnetic actuators can be independent of each other. However, for ease of manufacture and efficiency, it is beneficial for the ferromagnetic areas of each member to be part of the same ferromagnetic component.
It is also clear that the magnetic bearing is easier and less costly to manufacture if the two separate members have the same geometry. Furthermore, controlling the specific windings of the magnetic bearing is easier if the areas of each member are disposed symmetrically with respect to the transverse plane crossing the space between the separate members.
With the same aim of simplicity, the number of ferromagnetic areas of each member is advantageously an even number and each ferromagnetic area is preferably disposed opposite another ferromagnetic area with respect to the reference axis. In one particularly simple arrangement each member has four ferromagnetic areas divided into two pairs of areas which are diametrally opposed with respect to the reference axis and offset by 90xc2x0 about the reference axis.
The hollow outer part can have a large angular amplitude, for example plus or minus 50xc2x0 relative to the transverse plane perpendicular to the reference axis.
Each ferromagnetic area of each member preferably has first and second projections directed towards the inside surface of the hollow outer part to form the air-gaps of that area with the first of the projections surrounded by the specific winding.
The first or second projections of the ferromagnetic areas of a given member advantageously have free edges whose shape is a portion of a common sphere centered on the center of tilting, which guarantees that the air-gaps defined by the first projections likewise have the same radial thickness.
It is of course advantageous for the first and second projections to have edges formed by portions of a single sphere, so that all the air-gaps on one side of the transverse plane are equal. The projections of the two members preferably have edges on a common sphere so that all the air-gaps are normally equal, which facilitates determining and generating the flux variations needed for centering and tilting.
Two main configurations can be distinguished, according to whether the whole of the projections farthest from the transverse plane continuously face the inside surface of the hollow outer part or not, regardless of the inclination of the hollow outer part relative to the inner part, within the range of movement in tilting.
The simplest situation is that in which the above condition is satisfied.
In the contrary case, which is slightly more complex, the existence of air-gaps of varying section enables tilting torque to be generated by appropriate excitation of the windings without requiring components additional to those already mentioned.
The centering and tilting control functions are then obtained from a compact system which is contained within the overall size of the hollow outer part.
The projections in the ferromagnetic areas of a given member farthest from the transverse plane then advantageously have free edges whose shape is a globally trapezoidal portion of a common sphere centered on the center of tilting with each of the globally trapezoidal portions having one side extending to the vicinity of the reference axis. Allowing for the spaces which separate them circumferentially for fitting the windings, the second projections therefore conjointly form a spherical dome which maximizes the amplitude of the possible range of movement in tilting and guarantees that normally, in the correctly centered configuration of the hollow outer part, the air-gaps defined by the second projections and the inside surface of the hollow outer part are of the same thickness.
In this case, although it is not necessary for the second projections to be individually symmetrical about the transverse plane, for the two circular edges of the inside surface of the hollow outer part to have the same radius and for the projections at the ends to extend as far as the vicinity of the reference axis, it is nevertheless preferable for the two pluralities of second projections of the two members to be symmetrical from the point of view of their angular amplitude in a plane passing through the reference axis. This guarantees some degree of symmetry of tilting control performance.
In the first case, the projections farthest from the transverse plane are advantageously parts of a common annular projection, which helps to facilitate manufacture of the member. The annular projection is advantageously on the opposite side of the transverse plane crossing the space between the members to the first projections. Because the inner part has the overall shape of a ball, the specific windings can be located in a region of greater diameter which optimizes the number and size of the windings that can be mounted in the magnetic bearing.
In the first case, an additional winding is advantageously disposed around each member, between the first and second projections in the ferromagnetic areas. This provides a particularly simple way of generating flux lines distributed all around the reference axis, either in conjunction with the aforementioned specific windings or not.
Of course, these additional windings are connected to the excitation circuit, which is designed for selective application of excitation currents to the additional windings. In fact, the magnetic fluxes that such additional windings generate participate in centering the hollow outer part in the direction parallel to the reference axis. This is because the specific windings of each of the ferromagnetic areas can also be electrically energized to assure some or all of the centering parallel to the reference axis.
In each case, the specific windings of the ferromagnetic areas can simply be wound around one of the projections in the ferromagnetic area concerned. This projection can be the first or second projection in that area. However, for accurate control of the path of the flux lines, and therefore of the magnetic flux crossing each of the air-gaps, each ferromagnetic area includes two specific windings wound around the first and second projections, respectively.
The specific windings of the ferromagnetic areas of a given member are advantageously adjacent, at least in the circumferential direction, which maximizes the cumulative surface area of the free edges (and therefore the associated air-gaps) of the projections around which these windings are wound. If each ferromagnetic area has two windings, they are preferably also adjacent (in the direction parallel to the reference axis), which maximizes the cumulative surface area of the edges of the projections in that ferromagnetic area.
In the first case the hollow outer part can have a single opening to enable fastening of the members to an external frame and the hollow outer part intersects the reference axis on the opposite side of the single opening.
In the first case tilting control can be obtained by additional means. The tilting control means include, for example, externally of the hollow outer part:
two permanently magnetized rings which are carried by a first ferromagnetic armature attached to the hollow outer part around the reference axis, which each have a magnetization direction which passes at all points at least approximately through the reference axis, which are parallel to each other and offset in the direction parallel to the reference axis on respective opposite sides of the center of tilting and which have free edges substantially constituting portions of a common sphere centered on the center of tilting, and
an annular plurality of tilt windings which are attached to the inner part, which each include two groups of circumferential strands respectively adapted to face each of the permanently magnetized rings regardless of the orientation of the hollow outer part relative to the center of tilting within said range of angular movement in tilting of at least 5xc2x0 and which are carried by a second ferromagnetic armature defining in conjunction with the magnetized rings air-gaps whose thickness remains constant throughout said range of angular movement in tilting.
Clearly, a constant air-gap between the second ferromagnetic armature and the free edges of the magnetized rings guarantees constant tilt control performance within a range of movement in tilting of at least 5xc2x0. Also, the magnetization direction of the rings is preferably locally directed towards the center of tilting, which has the advantage that it maximizes the flux lines crossing the air-gap.
The two permanently magnetized rings preferably have the same diameter and are preferably symmetrical to each other about the center of tilting and the tilt windings are preferably symmetrical about the transverse plane. This facilitates the manufacture of the tilt control means. In practice, this symmetrical positioning of the tilt control means makes the range of movement in tilting symmetrical about the transverse plane passing through the center of tilting.
The spacing of the two rings in a plane containing the reference axis preferably corresponds to an angular offset of at least 10xc2x0 relative to the center of tilting. In the aforementioned situation in which the rings are symmetrical about the transverse plane, this amounts to the two rings defining an angular offset of at least xc2x15xc2x0 relative to the transverse plane passing through the center of tilting.
To maximize the efficacy of the tilt control means, the circumferential strands of each group of windings are disposed adjacently on a spherical surface of the second armature concentric with the center of tilting. This side-by-side disposition of the circumferential strands of each group has the advantage of minimizing the thickness of the air-gaps in which those strands are located.
It has been stated that the circumferential strands of the tilt windings continue to face the magnetized rings throughout the range of movement in tilting. To this end, the rings can have an angular amplitude in a plane passing through the reference axis less than that of each group of circumferential strands of each winding. In this way, in any tilting configuration, the whole of the free edge of the magnetized rings faces the circumferential strands. However, in a different embodiment, the rings have an angular amplitude in this plane passing through the reference axis which is greater than that of each group of circumferential strands of each winding, in which case the circumferential strands of each group remain at all times within one of the air-gaps defined by one of the annular rings.
The second armature (that carrying the tilt windings) is preferably disposed radially between the hollow outer part and the first armature (that carrying the magnetized rings).